Stoker-fired boiler, a method of modernization of stoker-fired boilers and a method of elimination of uncontrolled leakages of air not taking part in the combustion process in a stoker-fired boiler

ABSTRACT

A boiler according to the invention contains an additional air channel ( 9 ) with an inbuilt fan ( 10 ) with adjustable output. The first end ( 91 ) of the additional air channel ( 9 ) is connected with the under-stoker space ( 11 ) located outside of the wind boxes ( 5 ) of the under-stoker wind boxes system ( 4 ). The other end ( 9′ ) of the additional air channel ( 9 ) is connected with either the space of the channel ( 6 ) supplying outside air to the wind boxes ( 5 ) or with the atmosphere or with the secondary air channel of the combustion chamber ( 2 ). Modernization of the existing boilers consists in installation in the described above method of an additional air channel ( 9 ) with an inbuilt fan ( 10 ). The method of elimination of uncontrolled leakages in a stoker-fired boiler consist in leading out and air, getting through air gaps, from the under-stoker space of the boiler located outside of the wind boxes ( 5 ) to the described above additional air channel ( 6 ), wherein current output of the fan ( 10 ) built into this channel is proportional to the current difference between temperature of air supplied to the wind boxes ( 5 ) and temperature of air in the additional air channel ( 6 ).

TECHNICAL FIELD

The subject of the invention is a stoker-fired boiler containing in thecombustion zone a movable stoker and an under-stoker wind boxes system,a method of modernization of such a boiler as well as a method ofelimination of uncontrolled leakages of air not taking part in thecombustion process in the boiler.

BACKGROUND ART

In the commercial power industry and heating industry, for generatingheat stoker-fired boilers are commonly employed, in which the combustionof solid fuel, in particular of coal, takes place on a moving mechanicalstoker. Above the stoker there is an combustion chamber with a secondaryair channel, whereas under the stoker there is an under-stoker spacecontaining the under-stoker wind boxes system with wind boxes, and achannel supplying the air from the atmosphere to the inside of the windboxes, which is necessary for the combustion process on the stoker totake place. During utilization of the boiler, in the wind box spaces,the air pressure usually amounts to approximately 200-500 Pa. In theremaining part of the under-stoker space, the pressure is close to thepressure in the combustion chamber, and usually amounts to some −30 to−50 Pa. The appearing differences in pressure between the wind boxspaces, the under-stoker space, and the atmosphere cause flows ofuncontrolled leakages of air streams through the gaps existing betweenthese spacer, however prober manufacturing of a boiler does not ensureits complete tightness. Sealing of the said spaces in a stoker-firedboiler, meaning the elimination of uncontrolled leakages of air nottaking part in the solid fuel combustion process, was subject of manysolutions and attempts. From the Polish patent No. 183654 description,it is known that a stoker-fired boiler burner has already been sealed bymeans of metallic membrane walls. Migration of air streams within astoker-fired boiler may also be controlled through the appropriateshaping of wind boxes of the under-stoker wind boxes system. An exampleof a solution for the construction of an under-stoker wind boxes system,reducing the coefficient of excess air, was revealed in Polish patentapplication number P-355555.

DISCLOSURE OF INVENTION

The purpose of the invention was to reduce uncontrolled flows of airstreams within a stoker-fired boiler, negatively affecting itsperformance.

A boiler according to the invention contains an combustion chamber witha secondary air channel, a movable mechanical stoker, and anunder-stoker space containing an under-stoker wind boxes system withwind boxes, as well as a channel supplying the air into the inside ofthe wind boxes The essence of the invention is that the boiler containsan additional air channel with an inbuilt fan with adjustable output.One end of this additional air channel is connected with theunder-stoker space located outside of the wind boxes of the under-stokerwind boxes system.

In other embodiments of the boiler according to the invention, the otherend of the additional air channel is connected with either the space ofthe channel supplying the external air to the wind boxes of theunder-stoker wind boxes system, or with the atmosphere or with thesecondary air channel of the combustion chamber.

In another embodiment of the boiler according to the invention, in thespace of the channel supplying the air from the atmosphere to the insideof the wind boxes as well as in the space of the additional air channelis located at least one air temperature sensor. These sensors may beconnected to controller of the fan of the additional air channel.

A method of modernization according to the invention consists in thatthe under-stoker space of the aforementioned boiler, located outside ofthe wind boxes of the under-stoker wind boxes system, is connected withthe first end of the additional air channel having an inbuilt fan withadjustable output.

In other embodiments of the method of modernization, the other end ofthe additional air channel is connected with either the space of thechannel supplying the air to the wind boxes of the under-stoker windboxes system, or with the atmosphere or with the secondary air channelof the combustion chamber.

In another embodiment of the the method of modernization, in the spaceof the channel supplying the air from the atmosphere to the inside ofthe wind boxes, as well as in the space of the additional air channel islocated at least one air temperature sensor. These sensors may beconnected to the controller of the fan of the additional air channel.

The method of elimination of uncontrolled air leakages according to theinvention consists in leading out and air from the under-stoker space ofa boiler, located outside of the wind boxes of the under-stoker windboxes system, to an additional air channel having an inbuilt fan withchangeable output. Current output of this fan is proportional to thecurrent difference between temperature of air supplied to the windboxes, and the temperature of air in the additional air channel.

In other embodiments of the method of elimination of uncontrolled airleakages, the air led out to the additional air channel is eithersupplied to the wind boxes of the under-stoker wind boxes system, or isled out further into the atmosphere, or supplied to the secondary airchannel of the combustion chamber of the boiler.

The invention reduces the amount of air coming through the gaps andcombining with the combustion fumes, giving in consequence a reducedcoefficient of excess air in the fumes. This results in a reduction of aphysical loss in the fumes, which also means the increase of boilerperformance, usually by about 2 to 5%, depending on its technicalcondition, changes of the load, quality of the combustion charge, andproper maintenance and operation. The application of the invention alsoreduces the consumption of electric power by the components of theboiler by approximately 10-20%. As a result of general reduction of theamount of fumes removed from the boiler, the load of the extractor fanis smaller, and the consumption of energy by the fan of the additionalair channel is balanced by limited energy consumption by the wind boxesair fan. A beneficial feature of the invention is also a short timeneeded for the modernization of the boiler, and a low cost of suchmodernization that amounts to ca. 10% of the cost of replacement of theunder-stoker wind boxes system.

It unexpectedly appeared that a basic advantage of the invention issignificant reduction of dust emission (20-70%), which gives hope forthe possibility of meeting the requirements of future emission standardswithout installation of expensive electrostatic dust removers(dedusters). It also unexpectedly appeared that in the operation of theboiler with the invention being employed, emission of carbon oxide (CO)is reduced by 40-80%. This makes it possible not to equip boilers withsecondary air fans, and reduces consumption of electricity.

BRIEF DESCRIPTION OF DRAWINGS

The invention has been shown in a drawing, presenting schematic verticalcross sections of a stoker-fired boiler, whereas

FIG. 1 presents a boiler with the first variant of an additional airchannel,

FIG. 2 presents a boiler with the second variant of this channel, and

FIG. 3 presents a boiler with the third variant of such a channel.

MODE FOR CARRYING OUT THE INVENTION

A typical stoker-fired boiler has a fume zone with a fume extractor fan1, and a burner zone. In the burner zone there is an combustion chamber2, a movable mechanical stoker 3, an under-stoker wind boxes system 4with wind boxes 5, and a channel 6 supplying the air from the atmosphereto the inside of the wind boxes 5 by means of a wind boxes air fan 7.The combustion chamber 2 has a secondary air channel powered by a fan 8.The boiler according to the invention has an additional air channel 9with an inbuilt fan 10 with adjustable output. One end 9′ of theadditional air channel 9 is connected with the under-stoker space 11located outside of the wind boxes 5 of the under-stoker wind boxessystem 4. The other end 9″ of the additional air channel 9 may beconnected with either the space of channel 6 supplying external air tothe wind boxes 5 of the under-stoker wind boxes system 4, or with theatmosphere or with the secondary air channel of the combustion chamber2. Through controlling the output of the fan 10, an appropriate amountof air deriving from uncontrolled leakages is removed from theunder-stoker zone 11, and directed to a selected place via theadditional air channel 9. Output of the fan 10 may be controlledmanually, whereas it is beneficial to use for this purpose the knowledgeof air temperature in the space of channel 6 supplying it from theatmosphere to the inside of the wind boxes 5, and temperature of the airin the space of the additional air channel 9. In order to do so, inthese places at least one air temperature sensor (not shown in thedrawings) should be placed. For typical stoker-powered boilers, thedifference between the temperature of wind boxes air stream in thechannel 6, and temperature of air stream in the additional air channel 9not exceeding 5° C. shows that through channel 9 flows only the aircoming from uncontrolled leakages to the under-stoker space. Thedifference in temperatures amounting to 5 to 10° C. constitutesinformation that in the additional air channel 9 there also is the airderiving from uncontrolled leakages taking place through the section ofthe stoker 3 on which the combustion process no longer takes place ortakes place into a limited extent. In this case, uncontrolled leakagestake place towards the bottom part of the combustion chamber 2, andtheir reception has positive effects on the physical loss in the fumes.The difference in temperature exceeding 10° C. constitutes informationthat into the channel 9 additionally flow some of the fumes from thecombustion chamber 2 without resulting in a physical loss in the fumes.These dependencies allow for automation of the adjustment of output ofthe fan 10, through connecting of the said temperature sensors to thefan controller, and the application of the difference between thetemperatures measured by means of these sensors as a control parameter.It unexpectedly appeared that with small loads of the boiler, amountingto for instance 10-20% of the nominal load, the amount of air comingfrom the gaps is sufficient to conduct the process of combustion. Insuch a situation, when there is no possibility to isolate the channel 6from the atmosphere, the wind boxes air fan 7 stops supplying air forthe combustion process and only starts to block, the out-flow of airfrom the boiler. Elimination of air coming through the gaps from thestream of fumes in most cases causes the situation that the natural pullof the chimney suffices to ensure the appropriate negative pressure inthe combustion chamber 2, causing purposelessness of operation of thefumes extractor fan 1. However, according to the mandatory regulations,boiler operation without an extractor fan 1 is not permitted. A solutionto this problem may be operation with lower pressure value (e.g. −50 to−80 Pa in the combustion chamber 2), and limitation of output of theextractor fan 1. Very important for proper utilization of the boilerappeared to be the point of connection of the space of the channel 6supplying wind boxes air with the space of the additional air channel 9.Connection at the point which causes the situation that most of the airsupplied by the fan 10 reaches the front wind boxes 5 is unbeneficialfor functioning of the boiler, particularly with the aforesaidtemperature difference exceeding 10° C., because this means that anadditional stream of air is supplied with a lower oxygen content. Thesame stream of air directed to the last operating wind boxes 5positively affects the parameters of boiler operation, also with thetemperature difference exceeding 10° C. This is connected with lowintensity of the combustion process in this part of the stoker, meaningat the same time lower demand for oxygen. It unexpectedly appeared thatconnecting of the under-stoker space 11 through an additional airchannel 9 with the atmosphere does not require changes of the insofarapplied operation of the boiler (manual or automated). With theunder-stoker space 11 connected with the channel 6 or the secondary airchannel of the combustion chamber 2, the mutual interaction of thestreams of air, in particular with the temperature difference exceeding5° C., changes the insofar existing course of the combustion process,causing the need for measuring the streams of air and changing theboiler control algorithms. However, economic and ecological benefitsresulting from lower electricity consumption, and the use of heat of thestream of air from the gaps, justify additional outlays incurred on theimplementation of this solution, instead of a simpler to controlconnection of the under-stoker space 11 with the atmosphere. It alsounexpectedly appeared that the application of the invention givesadditional operation benefits. Connecting of the first wind boxseparated from the stream of wind boxes air with the under-stoker spaceallows boiler operators to control the distance of the point of coalignition on the stoker from the slide gate and the front part of theboiler structure. Similarly, connecting of the last non-operating windboxes with the under-stoker space after prior separation from the streamof wind boxes air reduces uncontrolled leakages from the wind boxesspace to the combustion chamber.

According to the invention, it is possible to build new boilers, as wellas quickly, cheaply and effectively modernize the existing ones. Forexample, modernization of a popular boiler type WR-10 entailsinstallation of an additional air channel 9 with a centrifugal fan typeWWOax-50 (made by Owent Olkusz) equipped with a 5.5 kW motor, whoserotation speed is controlled by means of a frequency converter. Withmanual control of this fan, decrease of stream of air supplied to theboiler was obtained by 6000 m³/h on average, with the boiler load within30 to 80%. The application of both fans type WWOax-50 for modernizationof a double-stoker boiler type WR-25 with the same manner of control ofthe fans caused a situation that within the load range of 40 to 80%, theexcess air coefficient in the fumes dropped from the level of 2.0-2.5 toapproximately 1.4-1.7. Temperature of combustion fumes after the boilerwas reduced by 15 to 20° C., dust content in theses fumes stream wasreduced by 20 to 70% and content of carbon oxide (CO) was reduced by40-80%.

1. A stoker-fired boiler having a combustion chamber (2) with asecondary air channel, movable mechanical stoker (3), and theunder-stoker space (11) containing an under-stoker wind boxes system (4)with wind boxes (5), as well as a channel (6) supplying the air from theatmosphere to the inside of the wind boxes (5), characterized in that itcontains an additional air channel (9) with an inbuilt fan (10) withadjustable output, wherein the first end (9′) of the additional airchannel (9) is connected with the under-stoker space (11) locatedoutside of the wind boxes (5) of the under-stoker wind boxes system (4).2. The boiler according to claim 1, characterized in that the other end(9″) of the additional air channel (9) is connected with the space ofthe channel (6) supplying outside air to the wind boxes (5) of theunder-stoker wind boxes system (4).
 3. The boiler according to claim 1,characterized in that the other end (9″) of the additional air channel(9) is connected with the atmosphere.
 4. The boiler according to claim1, characterized in that the other end (9″) of the additional airchannel (9) is connected with the secondary air channel of thecombustion chamber (2).
 5. The boiler according to any one of claims 1to 4, characterized in that in the space of the channel (6) supplyingthe air from the atmosphere to the inside of the wind boxes (5) as wellas in the space of the additional air channel (9) is located at leastone air temperature sensor.
 6. The boiler according to claim 5,characterized in that the air temperature sensors are connected tocontroller of the fan (10) of the additional air channel (9).
 7. Amethod of modernization of a stoker-fire boiler containing a combustionchamber (2) with a secondary air channel, a movable mechanical stoker(3), and an under-stoker space (11) containing an under-stoker windboxes system (4) with wind boxes (5), as well as a channel (6) supplyingthe air from the atmosphere to the inside of the wind boxes (5),characterized in that the under-stoker space (11) located outside of thewind boxes (5) of the under-stoker wind boxes system (4) is connectedwith the first end (9′) of the additional air channel (9) having aninbuilt fan (10) with adjustable output.
 8. The method of modernizationof a boiler according to claim 7, characterized in that the other end(9″) of the additional air channel (9) is connected with the space ofthe channel (6) supplying the outside air to the wind boxes (5) of theunder-stoker wind boxes system (4).
 9. The method of modernization of aboiler according to claim 7, characterized in that the other end (9″) ofthe additional air channel (9) is connected with the atmosphere.
 10. Themethod of modernization of a boiler according to claim 7, characterizedin that the other end (9″) of the additional air channel (9) isconnected with the secondary air channel of the combustion chamber (2).11. The method of modernization of a boiler according to any one ofclaims 7 to 10, characterized in that in the space of the channel (6)supplying the air from the atmosphere to the inside of the wind boxes(5) as well as in the space of the additional air channel (9) is locatedat least one air temperature sensor.
 12. The method of modernization ofa boiler according to claim 11, characterized in that the airtemperature sensors are connected with controller of the fan (10) of theadditional air channel (9).
 13. A method of elimination of uncontrolledleakages of air not taking part in the combustion process in astoker-fired boiler containing a combustion chamber (2), movablemechanical stoker (3), and the under-stoker space (11) containing anunder-stoker wind boxes system (4) with wind boxes (5), as well as achannel (6) supplying the air from the atmosphere to the inside of thewind boxes (5), characterized in that the air is led out from theunder-stoker space (11) located outside of the wind boxes (5) of theunder-stoker wind boxes system (4) to the additional air channel (9)with an inbuilt fan (10) with adjustable output, wherein current outputof this fan (10) is proportional to the current difference betweentemperature of air supplied to the wind boxes (5) and temperature of airin the additional air channel (9).
 14. The method of elimination ofuncontrolled leakages according to claim 13, characterized in that theair led out to the additional air channel (9) is supplied to the windboxes (5) of the under-stoker wind boxes system (4).
 15. The method ofelimination of uncontrolled leakages according to claim 13,characterized in that the air led out to the additional air channel (9)is led out further to the atmosphere.
 16. The method of elimination ofuncontrolled leakages according to claim 13, characterized in that theair led out to the additional air channel (9) is supplied to thesecondary air channel of the combustion chamber (2).